This invention relates to an instrument for making reflective measurements on a sample at different wavelengths and, more particularly, to an instrument in which the sample is illuminated by an integrating sphere and the light reflected from the sample is separated into its spectral components by a monochromator.
The purpose of using an integrating sphere to illuminate a sample in a reflectance measuring instrument is to provide uniform diffuse illumination of the sample. The integrating sphere receives light from a light source through an entrance port and the diffusely reflecting interior walls of the integrating sphere reflect the light in multiple reflections so that uniform diffuse illumination is provided over the interior surface of the integrating sphere. As a result, a sample positioned at a sample port in the sphere will be illuminated with this uniform diffuse illumination. A sample beam exit port is positioned to receive diffusely reflected light from the sample and the light passing through the sample beam exit port can then be separated into its spectral components to provide reflectance measurements of the sample at each wavelength. When an integrating sphere is used to provide the illumination to a sample, the color of the sample itself will cause a diminution and discoloration of the uniform illumination over the interior of the sphere and there is a need to correct the reflectance measurements for this diminution and discoloration. This correction can be provided by providing a second exit port in the sphere, called a reference beam exit port, positioned to receive light reflected from the wall of the sphere. The light reflected from the wall of the sphere through the reference beam exit port provides a measurement for comparison with the reflectance measurements made from the sample.